Advances in electronics technology have enabled the design and cost-effective fabrication of portable electronic devices. Thus, usage of portable electronic devices continues to increase as do the number and types of products. Examples of the broad spectrum of portable electronic devices include pagers, cellular telephones, music players, calculators, laptop computers, and personal digital assistants, as well as others.
The electronics in a portable electronic device generally require direct current (DC) electrical power. Typically, one or more batteries are used as an energy source to provide this DC electrical power. Ideally, the energy source would be perfectly matched to the energy requirements of the portable electronic device. However, most often the voltage and current from the batteries are unsuitable for directly powering the electronics of the portable electronic device. For example, the voltage level determined from the batteries may differ from the voltage level required by the device electronically. In addition, some portions of the electronics may operate at a different voltage level than other portions, thereby requiring different energy source voltage levels. Still further, batteries are often unable to respond quickly to rapid fluctuations in current demand by a device.
A typical arrangement is shown in FIG. 1 for an electronic device 10, such as portable computer, that includes an energy source 12, such as one or more batteries, and a load device 14, such as the internal electronics that require electrical power. Interposed between the energy source 12 and the load device 14 is a power supply 16 that may perform a number of functions. For example, a power converter 20, depicted as integral to the power supply 16, provides the necessary changes to the power from the energy source 12 to make it suitable for the load device 14.
The power supply 16 may also perform functions other than power conversion. For example, protecting the energy source 12, load device 14 and/or power converter 20 from damage by a sustained high electrical current may require electrically disconnecting the energy source 12 from the rest of the portable electronic device 10. As another example, the power converter 20 may require assistance during start-up which is provided by the supply 16.
With respect to the types of power conversion required, the power converter 20 may “step up” (i.e., boost) or “step down” the voltage. That is, the converter 20 may increase or decrease the input voltage VS from the energy source 12 across a pair of input terminals 24, 25 to an output voltage VO provided to the load device 14 across a pair of output terminals 26, 27. The power converter 20 may also store an amount of energy to satisfy a brief spike or increase in demand by the load device 14 that the energy source 12 is unable to provide.
The power converter 20 may also regulate the output voltage VO, keeping it close to the desired output voltage level and reducing rapid fluctuations that may cause detrimental noise or cause undesirable performance of the load device 14. Such fluctuations may occur due to changes in demand by the load, induced noise from external electromagnetic sources, characteristics of the energy source 12, and/or noise from other components in the power supply 16.
Inductive DC-DC power converters are often used in medium to medium/high capacity switching power supplies. Known inductive DC-DC power converters are based upon switching an output stage between a charge and discharge state. The output stage includes a switch that, when closed during the charge state, causes an inductive element, such as an inductor to charge (i.e., to store energy in an electric field) from the energy source. A rectifying element, such as a diode, is non-conductive, thereby preventing discharging to a load capacitor across the output terminals. During the discharge state, the switch is opened and the rectifying element conducts allowing the inductor to discharge into the load capacitor.
Known inductive DC-DC power converters are configured in various ways in order to achieve greater capacities, voltage ranges, and inverting/noninverting outputs. An inverted output has the opposite algebraic sign as the input. For example, an input voltage is provided at the positive input terminal 24 at +1.5 V referenced to a grounded negative input terminal 25. The positive output terminal 26 is grounded and the negative output terminal 27 is −1.0 V. Examples of known configurations include converters referred to buck, boost, buck-boost, noninverting buck-boost, bridge, Watkins-Johnson, current fed bridge, uk, single-ended primary inductance converter (SEPIC), buck square.
Inductive DC-DC power converters are often chosen due to power efficiencies which are greater than other converters such as linear converters, whose efficiency is related to the ratio of output voltage VO to input voltage VS. Also, the output voltage VO of inductive converters is generally related to the duty cycle of the switching, rather than the operating frequency of the switching, unlike generally known capacitive power converters.
However, known output stages for inductive DC-DC power converters 20 do have some drawbacks related to the capacitor, switch, and rectifying elements used in the converter. Specifically, reliance upon a diode as the rectifying element imposes a voltage drop across the diode that makes low input voltages (e.g., sub-one volt) impractical. In addition, generally known switches similarly require a control signal of a magnitude unsuitable for low input voltages. In addition, the range of practical inductance and capacitance values is constrained by achievable operating frequencies of the controller. Therefore, relatively expensive, noisy, and relatively large discrete inductors are required for the power output stage within an inductive converter.
Furthermore, known inductive DC-DC power converters 20 rely upon oscillator-based control. The inductor-capacitor combination chosen for these known “oscillator-controlled power converters” 20 generally dictate an operating frequency suitable for operation. Adjustments to the power delivered by the oscillator-controlled power converter is often provided by Pulse Width Modulation (PWM) or Pulse Frequency Modulation (PFM) by a controller. The problems with PWM and PFM schemes include circuit and fabrication complexity. Such complexity results in difficulty in miniaturizing the power converter 20 due to the number of discrete components necessary and/or the required area allocated on a semiconductor device.
In addition to the drawbacks associated with their complexity, oscillator-controlled power converters are also inefficient with light loads due to the continued operation of the oscillator.
Still further drawbacks in the prior art are the result of some inductive DC-DC power converters 20 using feedback, either inductor voltage VL or inductor current iL, feedback to sense the energy stored in the inductor as well as to sense the output voltage VO. These feedback techniques cause problems due to the nature of PWM and PFM control. For instance, inductor voltage VL feedback is an indirect approach to sensing the stored energy in the inductor L and introduces noise into the feedback voltage VF, (which is the same as or directly related to the inductor voltage VL,) due to fluctuations in input voltage VS and/or demand by the load device 14. Using current feedback avoids sources of voltage noise; however, known current-feedback power converters 20 suffer problems with respect to inadequate robustness to noise disturbances in the current feedback iF, (which is the same as or directly related to the inductor current iL,) resulting in premature switching and reduced power converter stability.